looking at the info apport provided I noticed that the write_limit is set to -1. The workaround is to delete the ~/.config/ubuntuone/syncdaemon.conf and restart the client. You can do that by opening a Tereminal (Applications > Accessories > Terminal) and running the commands:
u1sdtool --start && u1sdtool --connect
Also make sure you have run your updates and have the newest available client for Lucid because it looks as though this bug was fixed.